The interaction of cadmium with metallothionein, a low molecular weight cysteine-rich cadmium binding protein, will be investigated by 113Cd, 13C, and 1H nuclear magnetic resonance (NMR). This multinuclear NMR study should provide the detailed structural information needed to define the chemical nature of the cadmiun binding sites. Since metallothionein is suspected to play an important, though poorly characterized, physiological role in metal detoxification, metabolism and/or transport, such structural information may be expected to lead to an understanding of the nature of cadmiun toxicity and the mechanisms by which cadmium is handled by various organisms, including man, in the biosphere. The experimental approach will be to completely define the structural state of native rabbit liver metallothionein in terms of its natural abundance 113Cd, 13C, and 1H NMR parameters. Native metal ions will then be removed and substituted with enriched 113Cd(II) under conditions which maintain the native protein structure and cysteine oxidation state as monitored by 13D and 1H NMR. The extreme sensitivity of the 113Cd chemical shift to the nature and geometry of the surrouding ligands should allow a determination to be made of metal binding stoichiometry, extent of sulfur ligation, and symmetries of the individual binding sites. Dynamic information such as metal and ligand exchange rates will also be accessible. To explore questions relating to the functions of metallothionein, the rabbit liver protein will then be compared with protein from different tissue and species.